Cassava is a primary food source for more than 250 million Africans (about 40% of the continent's population). Its starchy root is a substantial portion of the diet of nearly 600 million people worldwide. In fact, cassava is relatively easy to grow in drought conditions and its root can stay in the ground for up to two years.

Cassava mosaic disease (CMD) is the most important disease of cassava in Africa and the Indian subcontinent. The disease is caused by cassava mosaic viruses of the genus Begomovirus (family Geminiviridae) that are transmitted by the whitefly, Bemisia tabaci (Gennadius), (Homoptera, Aleyrodidae). For more on this insect, please refer to the January 2007 APSnet.org feature.

Three approaches to control the spread of the cassava mosaic virus are commonly considered. The first is the use of insecticides, with potential impacts on non-target organisms. The second is the use of cultivars with cassava mosaic virus resistance. In general, breeding resistant cultivars takes several years. The third method is to intercrop cassava with other crops such as corn or cowpea.

Fondong et al. (2002) studied the spread of cassava mosaic disease, both in cassava alone and cassava intercropped with corn, cowpea, or both corn and cowpea. After a two-year experiment, they found that the disease progress curves in the three intercropped treatments had a similar shape and were less sigmoid than the curve for cassava grown alone.

Below is a comparison of curves for disease progress under monocropping and intercropping illustrated using R.

Output

Based on these results, Fondong et al. (2002) concluded that intercropping decreased the number of whiteflies and reduced the disease incidence, whereas intercropping did not affect disease severity. Reducing the number of whiteflies may also slow spread of the disease to adjacent areas.